PDDL file icons

[jan-dolejsi]

It looks like the file icons could be also embedded into this extension (reducing the dependency on other extensions), plus it supports light and dark theme.

https://code.visualstudio.com/api/extension-guides/file-icon-theme

[evelynmitchell]

It appears that this plugin requires https://github.com/vscode-icons/vscode-icons but it isn't installed with this plugin.